1. Field of Use
This invention relates generally to telescopic booms such as are used on mobile cranes or the like. In particular, it relates to the construction of such telescopic booms and boom sections therefor.
2. Description of the Prior Art
Current practices in the construction industry and building trades require mobile cranes having telescopic booms which can handle increasingly heavier loads and raise them to greater heights. Attempts to increase boom size, load-handling capability, and strength merely by enlarging the size of prior art designs have not been entirely successful. As the physical size of such telescopic booms and the boom sections used therein are increased, and as the lengths to which they can be extended are increased, the booms become extremely heavy and awkward to operate. Further, the excessive deflections due to higher forces hinder precise load handling and cause undesirable whiplash. Various approaches in boom design and construction have been employed to achieve greater size and strength without suffering undue weight penalties. For example, lattice-type booms are sometimes employed and attempts have been made to lighten the boom by piercing holes in the heavy gauge sheet steel of which some boom sections are fabricated. Also, designs employing optimum selection and arrangement of boom section components have been employed to achieve high strength versus weight ratios.
Typical boom sections comprise rectangular, trapezoidal or triangular cross sections. Very often, in booms wherein the top, bottom and side walls are made of steel plate, high strength structural steel about 3/16 of an inch thick is the minimum thickness necessary in boom walls to obtain acceptable performance in respect to buckling, tensile, and compressive stresses. Often, such boom walls are provided with reinforcement bars, channels, and a multiplicity of stiffeners. Such construction has disadvantages. The walls or channels are most generally joined by means of welding processes which use filler material. Thus, structural integrity is achieved by depositing of some welding rod material. This is time-consuming, costly in terms of labor and material, and also adds to the weight of the boom. Typically, the welding material per se amounts to from 1/2 to 11/2% of the boom weight. Also, to accommodate sliding between the adjacent boom sections, bearing pads of various materials and configurations are used, such as rollers, bearings, Teflon slide or bearing pads. For practical design reasons, pads are limited in size. And, since the bearing loads are high and the load bearing surface is small, bearing pads are subject to high compressive stresses. Therefore, high wear occurs and thus adjustment means are required to maintain proper clearance between the boom sections.
The prior art contains numerous examples of prior art telescopic booms and the following U.S. patents show the state of the art: U.S. Pat. Nos. 4,004,695; 4,003,168; 3,931,698; 3,890,757; 3,719,403; 3,708,937; 3,690,742; 3,620,579; and 3,423,890. U.S. Pat. No. 3,931,698 discloses an improved crane boom structure for telescopic boom sections. The conventional, generally rectangular cross-sectional shape is modified to provide central guiding of the boom sections to locate the bearing forces transmitted between sections at or near the neutral axis where bending stresses are minimal for greater load carrying capacity for a given boom weight. The boom side walls are provided with channel-shaped projections, the top and bottom channel legs being located on either side of the neutral axis of the beam section, preferably symmetrical therewith. Bearing members are interposed between top and bottom channel legs of contiguous boom sections.